This invention relates to apparatus and methods for electroless plating on substrates. More specifically, the invention relates to improved apparatus for controlling engaging substrates during electroless plating in order to provide good plating performance at relatively high temperatures.
The Damascene process provides inlaid copper lines in dielectric layers of integrated circuits. The copper lines provide electrical routing (metal interconnects) between circuit elements in the integrated circuit. Damascene copper lines are rapidly replacing traditional aluminum etched lines in high-performance integrated circuitry.
Currently, a preferred method of metal-interconnect layer deposition is electroplating. This is in part due to the success of xe2x80x9cbottom-upxe2x80x9d copper filling methods for damascene features. The process typically involves formation of a barrier layer (typically composed of Ta, TiN or TiSiN) and a seed layer (typically copper) over the wafer, followed by plating the wafer to fill embedded structures from the bottom-up. A number of problems occur when trying to accomplish this task. Such problems include corrosion of the seed layer and associated reactions in the plating bath, poor structures (morphology) of PVD-deposited seed layers, non-uniform deposition of the metal over and into features, and shrinking of feature volume (and associated increase in aspect ratio) when seeded.
As features become smaller, the seed layer must become thinner (otherwise the feature will be closed off by the generally non-conformal PVD seeding process). However, most useful electroplating tool designs require supplying current to the wafer from the wafer""s outer edge via the seed layer. When attempting to electroplate using ever thinner seed layers for supplying plating current, the current distribution becomes increasingly dominated by the resistance in the seed layer. This phenomenon is commonly referred to as the xe2x80x9cterminal effect.xe2x80x9d
Thus, the need exists to find methods of depositing seed layers in a more conformal manner. This is because conformal seed layers reduce resistance by providing a greater average thickness in comparison to non-conformal layers deposited by PVD, for example. As a result, the terminal effect is mitigated during electroplating.
Electroless plating can provide highly conformal seed layers. And in some cases, electroless plating can replace not only PVD seed deposition, but electroplating as well, thereby dispensing with the need for a plating current and a seed layer. This, of course, circumvents the problems of the terminal effect and poor seed layer step coverage.
General process requirements for wafer plating include global and local plating uniformity, defect free process, and high throughput. In order to produce plating equipment for high-volume manufacturing (e.g., damascene copper processing) meeting these requirements, advances in electroless plating hardware design are required.
There are problems associated with available and proposed electroless plating apparatus. Most of these are due to heating. Electroless plating can take place at temperatures near room temperature, but to realize good performance for integrated circuit fabrication applications, it is preferably performed at higher temperatures, in the range of about 50 to 90 degrees C. As a consequence, electroless plating baths decompose to some degree during the plating process. In addition, conventional plating equipment can expand, deform, and/or stick to components during plating.
Some wafer electroplating apparatus can provide most of the functionality required for electroless plating. One example is the SABRE(trademark) xe2x80x9cclamshellxe2x80x9d electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. Pat. Nos. 6,156,167 and 6,139,712, which are herein incorporated by reference in their entireties. The clamshell apparatus provides many advantages for wafer throughput and uniformity; most notably, wafer backside protection from contamination during electroplating, wafer rotation during the electroplating process, and a relatively small footprint for wafer delivery to the electroplating bath (vertical immersion path). During plating, it compresses the wafer between a xe2x80x9ccupxe2x80x9d and a xe2x80x9ccone,xe2x80x9d thereby sealing off the backside of the wafer from contact with plating solution.
Modifications to the xe2x80x9cclamshellxe2x80x9d and its associated plating environment for improved wafer uniformity and quality have been described in U.S. Pat. Nos. 6,074,544, 6,110,346, 6,162,344, and 6,159,354 which are herein incorporated by reference in their entirety. The described modifications relate to methods for using variable currents, improved mass transfer, and electric potential shaping. Other documents providing details of the clamshell design include the following: U.S. patent application Ser. No. 09/927,741, filed Aug. 10, 2001, titled xe2x80x9cClamshell Apparatus For Electrochemically Treating Wafersxe2x80x9d, and naming Reid, et al. as the inventors; U.S. patent application Ser. No. 09/872,340, filed May 31, 2001, titled xe2x80x9cMethods and Apparatus for Bubble Removal in Wafer Wet Processingxe2x80x9d, and naming Patton, et al. as the inventors; U.S. patent application Ser. No. 09/872,341, filed May 31, 2001, titled xe2x80x9cMethods and Apparatus for Controlled-Angle Wafer Immersionxe2x80x9d, and naming Reid, et al. as the inventors; and U.S. patent application Ser. No. 09/927,740, filed Aug. 10, 2001, titled xe2x80x9cMethods and Apparatus for Controlling Electrolyte Flow for Uniform Platingxe2x80x9d, and naming Mayer, et al. as the inventors.
Many features of the basic clamshell apparatus are suitable for electroless plating. However, some features must be modified to account for the relatively high temperatures required for electroless plating.
The present invention addresses these needs by providing certain improved features of a clamshell plating apparatus designed for use at relatively high temperatures (e.g., at least about 50 degrees C.). It accomplishes this with judicious choices of construction materials and feature positioning. For example, the clamshell cup and cone components that engage the work piece are made from dimensionally stable materials with relatively low coefficients of thermal expansion. Further, O-rings are removed from positions that come in contact with the work piece. This avoids the difficulty caused by O-rings sticking to work piece surfaces during high temperature processing. In place of the O-ring, a cantilever member is provided on the portion of the cone that contacts the work piece. Still further, the invention may employ a heat transfer system for controlling the temperature of the work piece backside during plating.
One aspect of the invention pertains to apparatus for engaging a work piece during plating. Such apparatus may be characterized by the following features: (a) a cup having a circumferential side wall defining an interior region and a lip within the interior region arranged such that lip can support the work piece while the work piece remains within the interior region; (b) a cone having a work piece contact surface that fits within the cup""s interior and can contact the work piece in a manner that holds the work piece in a fixed position against the cup""s lip; and (c) a circumferential cantilever member provided on the contact surface, which cantilever member elastically deflects holding the work piece in the fixed position. Preferably, the cone""s circumferential cantilever member and the work piece contact surface form a monolithic element. That is, they are formed from a single piece of material.
Preferably, the cantilever member has certain associated features such as a contact feature and a backing O-ring. The contact feature protrudes from a distal end region of the circumferential cantilever member and points toward the cup. Thus, the contact feature is the first portion of the cone to contact the work piece during engagement of the work piece. Typically, the contact feature is implemented as a circumferential ridge on the circumferential cantilever member. The O-ring is located in a slot behind the cantilever member so that it can impart a return force when the work piece is held in the fixed position.
Another aspect of the invention pertains to methods of plating a material onto a work piece. Such methods may be characterized by the following sequence: (a) loading a work piece onto a cup as described above; (b) engaging an exposed surface of the work piece with a cone having a circumferential cantilever member as described above, which elastically deflects while holding the work piece in the fixed position; and (c) plating a metal onto the work piece while engaged by the cup and cone. Preferably, the plating is an electroless plating operation taking place at a temperature of at least about 50 degrees Centigrade. To this end, the method will also require and operation of immersing the work piece in a plating fluid prior to plating.
Another aspect of the invention pertains to apparatus for heating and/or cooling a work piece during plating. Such apparatus may be characterized by the following: (a) a cup as described above; (b) a cone having a work piece contact surface that fits within the cup""s interior and can contact the work piece as described above; and (c) a heating element in the cone. In this aspect of the invention, the cone""s contact region is shaped to form a heat transfer fluid pathway between the work piece and the contact surface when the work piece is in the fixed position. To facilitate use of the heat transfer fluid, the apparatus will typically include both a heat transfer fluid inlet and a heat transfer fluid outlet in the cone. These are configured to handle either or both of a liquid and a gas heat transfer fluid (e.g., helium).
Preferably, the cone""s contact region is shaped such that the heat transfer fluid pathway has a height of between about 100 and 500 micrometers. To produce the heat transfer fluid pathway, the cone""s contact region may include at least one protruding contact feature pointing toward the cup. This feature will be the first portion of the cone to contact the work piece during engagement of the work piece. Therefore, it keeps the remainder of the cone""s contact surface slightly separated from the work piece back side to define the heat transfer fluid pathway.
Yet another aspect of the invention pertains to methods of heating a work piece while plating a material onto that work piece. These methods may be characterized by the following operations: (a) loading the work piece onto a cup as described above; (b) engaging the exposed surface of the work piece with a cone as described (although the cantilever is not required); (c) plating a metal onto the work piece while engaged by the cup and cone; and (d) passing or maintaining a heat transfer fluid along the exposed surface of the work piece during plating. Preferably, the methods also heat the heat transfer fluid with a heating element provided within the cone.
The remainder of the specification will describe these and other features and advantages of the invention in further detail.